Prospecting Of Fine Crustal Structure And Research On Seismogenic Tectonics Of Linfen Basin

The Linfen basin is located in the south part of the Shanxi Rift Zone. In1303and1695, two great earthquakes with M8.0and M73/4occurred one after another in this basin. This phenomenon is extremely rare for the mainland that in less than400years two great earthquakes occurred in one basin. Therefore, the cause of formation, the seismogenic environment in the deep crust and the coupling relationship between shallow and deep structures of the Linfen basin still need to be studied further.In view of the above, we carried out the comprehensive research on the fine crustal structure, geodynamic process and deep seismogenic structure based on the work regarding deep seismic reflection, shallow seismic reflection with high resolutions and precise earthquake location.The study on the deep seismic reflection profiles reveals that for the upper, middle and lower crust the variation of structural feature in vertical direction is evidently different, indicating that the forces driving tectonic movement and deformation of curst structure come from the deep in the Linfen basin. This study also finds a mirror image relationship between the basin basement and variation in Moho interface. That is, the shallowest Moho is located under the middle part of the Linfen basin, indicating a pure shear extensional pattern of the Linfen basin. Resulted from by seismic materials, the crust mantle transition zone shows a feature of multi phases, complex strong reflection and laminated structure. This implies that a zone of2-3km high composed by high and slow velocity materials was developed at the base of the curst, reflecting the intrusion process of hot materials from upper mantle into the curst.Based on the tectonic phenomena revealed by deep seismic reflection and combining the geological and geophysical data available, we think that the Linfen basin is a typical’half graben’. A series of ancient crystalline metamorphic rocks underlying another series of west-dipping sediments with comfortable contact in the basin indicates this area has experienced sedimentary discontinuities and multi period tectonic activities. Additionally, the piedmont fault of Luoyun Mountain, Fushan fault and other6buried active faults since the Quaternary detected by seismic reflection profiles show "negative flower shaped’ structure distribution and they control the formation of crust structure and tectonics in the Linfen basin and its sedimentary.We relocated the earthquakes of1981-2013in the Linfen basin precisely by using both the absolute and double-couple location methods. The results show that the focal depth concentrates in two zones of5-11km and15-27km deep with high velocity speed, which implies that the two zones are the main seismogenic zones of the Linfen basin, and the generation and the occurrence of these earthquakes were mainly controlled by the Luoyunshan deep fault. This crust structure with high and low velocity layers in between, especially exist of low velocity layers and distribution depths, is the key factor that influences the generation of earthquakes. This further shows that the main force driving earthquake activities originates in tectonic activities in the deep crust and adjustment of crust structure caused by the uplifting of the asthenospheric materials.The last part of our work was focused on the research on the distribution characteristics of discovered deep buried faults in the Quaternary sediments and their activities in terms of the results of high-resolution shallow seismic prospecting and the conclusion from deep seismic reflection prospecting. The results show that the Fenxi fault, Fendong fault, Quting fault and Yandi fault, which cross the basin, all cut the early Quaternary strata in the shallow part and belong to the Quaternary buried active faults. Their deep parts have a consistently corresponding relationship between the revealed deep buried faults. Therefore, these faults should be paid more attention to for the future work of earthquake prevention and disaster reduction in the Linfen city.The results above depicted the fine crustal structure of the Linfen basin, the extension situation of active faults in the deep crust and the coupling relationship between the shallow and deep structures, and revealed the main seimogenic zones and seismotectonics of the study area. Our study may provide evidence from the angle of seismology for further research on mainland geodynamics.